Two important applications of interferometers are metrology and spectrometry. Spectrometry includes many types of measurements of wavelength and amplitude, as well as spectral imaging. One important commercial application of interferometers is spectroscopic measurements in the infrared spectral region. Fourier transform infrared (FT-IR) spectrometers are versatile and powerful tools for measuring infrared spectra. They have dominated the marketplace and the experimental literature of vibrational spectroscopy for almost three decades. Their success results largely from the well-known multiplex, throughput and registration advantages of interferometric spectrometry. Many aspects of the art of interferometry have been compiled. See, for example, P. R. Griffiths and J. A. de Haseth, Fourier Transform Infrared Spectrometry, (New York: John Wiley and sons, 1986).
In the usual rapid-scan mode of operation of FT-IR spectrometers, a collimated beam of radiation is intensity modulated by scanning one of the interferometer mirrors to produce a constant optical velocity on the order of 0.06 to 6 cm/s. The resulting modulated radiation intensity (an interferogram), possibly modified by interaction with a sample, is recorded from the output by an infrared detector. If a system under study varies with time, the time-scale of the spectral changes determines whether or not conventional rapid-scan FT-IR spectrometry may be used for the measurement. Typical FT-IR spectrometers require between 40 ms and 1 s to sweep an interferogram to 4-cm.sup.-1 resolution. If the spectral information varies with frequency components lower than the scan rate, the time-scale of the spectral variations is longer than the time-scale of the spectral measurement, i.e., the system is varying slowly with time. Under these conditions, the conventional rapid-scan mode of operation of FT-IR spectrometers may be conveniently used to generate a time-resolved sequence of spectra (interferograms). By increasing the scan rate of the interferometer mirror, the time required for spectral measurement can be shortened to increase the range of application.
One of the fundamental limitations, however, of conventional FT-IR instruments is the rate at which a spectrum (i.e., interferogram) can be scanned. For all commercial FT-IR instruments, the limitation arises principally from the linear motor used to drive interferometer scanning, together with the mass of the moving mirror assembly. The shortest possible scan time in most systems operating at 4-cm.sup.-1 resolution is approximately 40 ms. Under these conditions, the time required to turn around the mirror is comparable to the data acquisition time, i.e., the duty cycle efficiency is low, and drops further if the scan rate is increased.
Limited scan rates have been tolerated for so long for a number of reasons. As noted above, in conventional instruments the interferometer mirror is translated by a voice coil drive. The mirror sweep encodes the radiation intensity so that all wavelengths can be measured simultaneously (the Fellgett advantage). A Fourier transform is then necessary to decode the interferogram and recover the spectrum. In step-scan operation the time-dependence of the encoding is essentially removed by stopping the mirror, thus simplifying a variety of time-resolved spectroscopic measurements. The step-scan and stroboscopic approaches are applicable only to processes that can be repeated many times. These approaches are not suitable for real-time measurements.
An alternate approach uses very-rapid-scan operation in which the mirror is moved very quickly, so that a system under study remains in a nearly constant state for the duration of one scan. This approach has the added benefits of producing data in real time and avoiding susceptibility to low-frequency noise sources that adversely affect step-scan and stroboscopic FT-IR measurements.
The force developed by a voice coil mirror drive is the product of the magnetic field strength, the length of the winding, and the current in the winding. For a conventional FT-IR system the maximum force that can be developed is on the order of 1 or 2 N. The mass of the moving mirror and the moving portion of the bearing can be taken to be 30 g, but in some systems these have a mass greater than 500 g. A typical FT-IR system is barely capable of driving the mirror through a distance suitable for phase modulation at 1 kHz. This distance is only a few fringes of the reference laser, much less than the 4000 fringe sweep required for 4-cm.sup.-1 resolution. Individual systems may vary in mass, voltage and other details of their designs, but such differences do not matter much. At low frequencies, voice coil response drops as 1/f.sup.2 for purely mechanical reasons. At higher frequencies, the coil inductance becomes significant and the response drops as 1/f.sup.3. The acceleration that is required for measuring a 4-cm.sup.-1 spectrum in 1 ms can be readily derived and is about 12,500 m/s.sup.2. To achieve this acceleration a force of about 375 N must be applied to a 30 g mirror, with a resulting optical velocity of 250 cm/s. It might be possible to gain a factor of 100 in voltage (with a substantial high-voltage power supply and amplifier), and a factor of 10 in magnetic field strength, but there are fewer, if any, mirror/bearing assemblies that would withstand forces of this magnitude. Even if such an assembly could be constructed, the reaction force would tend to cause unacceptable disturbances to other optical components.
Moving a mirror with the velocities and accelerations suggested above would permit a very-rapid-scan operation to combine many of the advantages of conventional FT-IR with the advantages of step-scan operation. Such a system would have application to both repeatable and non-repeatable events on the millisecond time-scale. This approach would fail for transients which are too fast, and is generally not suitable for photoacoustic or photothermal measurements which are dependent on the modulation (Fourier) frequencies of the spectral multiplexing. The modulation frequencies produced are proportional to the velocity of mirror travel. An optical retardation velocity appropriate for use with a pyroelectric detector is about 0.3 cm/s, corresponding to a modulation frequency of 5 kHz for reference laser radiation at 15,804 cm.sup.-1. At this speed a single 4-cm.sup.-1 scan requires 840 ms. For a mercury-cadmium-telluride (MCT) detector, a much higher optical velocity, typically 6 cm/s, produces better results. The reference laser is then modulated at 100 kHz, but the scan is still 42 ms. If a 4-cm.sup.-1 interferogram is scanned at constant velocity in 1 ms, the optical velocity will be about 250 cm/s and the laser will be modulated at 4 MHz. This is 1000 times higher scan rate than is appropriate for a TGS detector, and a million times higher than is commonly used in step-scan measurements. The Fourier frequencies for the mid-infrared will be in the range from 100 kHz to 1 MHz under these conditions. As the mirror velocity changes, the dynamic range of the interferogram also changes. The amplitude of the optical interference signal remains constant, independent of scan velocity. If the detector response is flat to the highest frequency present, then the electronic interferogram amplitude is also independent of scan speed. More of the detector noise is passed as the detector bandwidth is opened to accommodate a broader range of modulation frequencies. The net result is that the SNR per scan goes down as the square root of the scan time, while the SNR per measurement time remains constant. Analog-to-digital converters that can sample with sufficient resolution to allow interferogram recording out to 100 MHz (were it possible to scan a mirror that rapidly) have been available for some time.
A rotary motion can be a better approach to generating rapid path difference variation. There is, however, an apparent symmetry mismatch between rotating optical elements and the nominally planar wavefronts in a Michelson interferometer. To reach a speed of 1 ms per scan, the rotating element will probably have to spin at 30,000 rpm. Under a stress of up to 50,000 g's they would have to be flat to approximately 250 nm, a tenth-wave of 2.5 .mu.m radiation. In spite of the apparent symmetry mismatch, numerous rotating interferometer designs have been proposed and demonstrated. Each of these designs exhibits some tradeoffs. Dybwad's design, disclosed in U.S. Pat. No. 4,654,530, uses a rotating prism to vary optical path difference. It has the advantages of simplicity, compactness, and generation of 4 scans per revolution. Disadvantages include a duty cycle well below 100% because of the limited range of angle that produces useful modulation, and dispersion in the prism which distorts the wave number axis. There may be cause for concern about the mechanical strength of suitable IR-transparent prism materials spinning at high speeds. The airflow around the flat surfaces may cause unacceptable drag and turbulence if the system is not evacuated.
Another approach to the symmetry problem is to use optical components of circular or spherical symmetry. M. Bottema and H. J. Bolle, in the Aspen Int. Conf. on Fourier Spectroscopy, have described a confocal interferometer design. 1970 (G. A. Vanasse, A. T. Stair, and D. J. Baker, eds. ), AFCRL-71-0019, p. 293 (1971). This design permits only a minute range of retardation. The wave fronts approaching the detector are converging such that the foci from the two arms of the interferometer will only match sufficiently well to produce useful interference for a very limited range of retardation. However, the spherical symmetry of the reflectors allows a large divergence angle in the input beam. Such spherical symmetry might allow the reflector to be a portion of a toroid that is rotated about its center. A confocal design described in U.S. Pat. No. 4,179,219, by Smith, allows for a much larger range of retardation. The convergence rate of the wave fronts is compensated by a varying focal length in the rotating reflector. This design has a more compact size and a larger acceptance angle. It is, however, quite difficult to fabricate a mirror of the required figure. A second drawback of this design is that the mirror figure is a compromise between compensating the focal length for path variation and the slope of the surface required for varying the path difference.
Another approach to generating path difference by rotating optical elements is the use of spinning cube-corner retroreflectors mounted off axis such that the shear varies with rotation angle. Each shear distance produces a unique path difference. Any beam impinging on the reflector will be reflected antiparallel to the incident beam regardless of the rotation angle or shear offset. A series of designs are disclosed by Tank, et al. One appears in U.S. Pat. No. 5,341,207. A variety of related configurations have been published, including the one in U.S. Pat. No. 5,148,235. Some of these designs utilize multiple spinning retroreflectors to increase the rate of change and maximum excursion of path difference. The advantages of these designs include intrinsic tilt- and shear-compensation. Problems with this approach include the fact that off-axis spinning of retroreflectors is inherently unbalanced, and above about 6000 rpm distortion of the retroreflectors becomes significant. Furthermore, large retroreflectors are fairly expensive.
The literature holds deeper insight. Kauppinen's paper, J. Kauppinen, I. K. Salomaa, and J. O. Partanen, Applied Optics vol. 34, p. 6081 (1995)!, discusses a series of related interferometers using the same approach to solve the symmetry problem: complementary reflections from two separate flat mirrors in each arm of the interferometer insure that a final reflection from a plane mirror is at normal incidence. This sends the beam exactly back to the beamsplitter via the inverse of its original path. Perkin-Elmer's Dynascan design is a variant of this general approach. Kauppinen's paper was particularly valuable because it showed enough variations on this theme to allow the common features to be extracted. It also clearly showed that exotic surfaces are not required to solve the symmetry problem, i.e., convert rotation to planar retardation. Kauppinen has proposed and demonstrated a variant which also uses planar reflectors with rotational motion. It is optically equivalent to both Perkin-Elmer's and Brierley's designs (vide infra), but with the advantage that it is particularly compact and resistant to deformation by mechanical and thermal stresses. None of these designs are intended for complete rotation.
Another variation on this theme is Brierley's U.S. Pat. No. 4,915,502. Brierley's design uses two parallel plane mirrors on a rotating platform. Although Brierley specifically excluded the case of complete rotation in his patent claims, the duty cycle for measuring 4-cm.sup.-1 interferograms using complete rotation of his apparatus is estimated to be 6%. Further, such a design would be difficult to balance for spinning, suffer serious distortion and have to be evacuated because the large flat surfaces would impede rotation in air.
In Brierley, the axis of rotation is a line coming straight out of the drawing. In FIG. 6 of Tank's disclosure in U.S. Pat. No. 5,457,529, the rotation axis is a vertical line in the plane of the paper. As a consequence, the maximum retardation is set by the tilt of the reflector assembly when it is fixed to the rotating shaft. Again, two separate reflectors are used to make the complementary reflections. The problem with this general design by Tank is that alignment between two fairly large spinning mirrors must be maintained to 0.5 arcseconds. The retardation and optical velocity vary sinusoidally with rotation angle. The resulting variations in the modulation frequencies are not a problem if the delays in the infrared and laser channels are matched A. S. Zachor, Applied Optics vol. 16, p. 1412 (1977), A. S. Zachor and S. M. Aaaronson, Aplied Optics vol. 18, p. 1345 (1977)!. Another approach is disclosed in FIG. 3 of U.S. Pat. No. 5,457,529. This design also uses two separate reflecting surfaces but transforms the problem of aligning two separate rotating reflectors into a problem of polishing two sides of the same parallel disk. However, this design has the disadvantage of adding folding mirrors. It may be easier to adjust the alignment of several static mirrors than the alignment of even one spinning mirror. Tank places an aperture in the center of the disk that makes it more difficult to mount and spin the disk. However, in compensation, Tank has a considerable advantage over a solid disk design which uses both sides of the disk. Because the beam can pass through the center of the disk, the path length is greatly reduced, and consequently a larger beam divergence can be tolerated.
One disadvantage of using the tilted disk of Tank to precess or nutate two parallel surfaces is that the tilt produces a strong bending moment in a centrifugal field. Finite element modeling of a beryllium disk of 1.25-cm thickness and 10-cm diameter tilted 1 degree relative to the shaft results in a deflection of approximately 600 nm. For an aluminum disk of equal dimension, the deflection would be about 6 times greater or nearly 3600 nm. It is desirable to maintain flatness to better than 250 nm for mid-infrared interferometry.
Several types of tilt-compensation optics are known. The most common approach in commercial interferometers is the use of feedback control to provide active tilt-compensation. The tilt of a fixed plane mirror is varied to compensate for tilt of a moving plane mirror. Nicolet and Digilab currently practice this method. A second commercial approach is the use of cube-corner retroreflectors. The simplest use of a cube-corner retroreflector provides tilt- but not shear-compensation. Mattson, Bomem and Oriel currently manufacture interferometers that incorporate cube-corner retroreflectors. Passive tilt compensation has also appeared numerous times in the prior art. Steel's design W. H. Steel, Aspen Int. Conf. On Fourier Spectroscopy, 1970 (G. A. Vanasse, A. T. Stair, and D. J. Baker, eds.), AFCRL-71-0019, p. 43 (1971)! also uses a cube-corner retroreflector. Another well-known general method is the use of cat's eye retroreflectors. Perhaps because of expense, cat's eye retroreflectors have appeared only in instruments intended for high-resolution measurements. Tilt compensation is important because tilt compromises photometric accuracy and modulation efficiency, particularly when large path differences or short wavelengths such as the near-infrared, visible or ultraviolet spectral range are used.
It is an objective of the present invention to tilt- and shear-compensate an interferometer. It is also an objective of the present invention to allow rapid variation of the path difference of an interferometer. It is also an objective of the present invention to measure multiple spectra rapidly, especially in a Fourier Transform-Infrared (FT-IR) spectrometer. It is also an objective of the present invention to measure one or more spectra rapidly with relatively high resolution. It is also an objective of the present invention to measure complete spectra faster than possible with most present spectrometers and especially FT-IR spectrometers.
The present invention contemplates a spectrometer having a source of a beam of radiant energy, a beamsplitter for dividing the beam of radiant energy into at least first and second energy beams, a first return mirror for reflecting the first beam of radiant energy back to the beamsplitter. A moving mirror receives the second energy beam from the beamsplitter and is capable of moving with some projection along the path of the second energy beam. The moving mirror has a planar optical surface that reflects the second energy beam. A retroreflector receives the second energy beam from the moving mirror and produces a reflection that is complementary at least in part of the second energy beam. A second return mirror returns the second beam of energy to the beamsplitter along a path that is antiparallel to at least a part of the path taken by the second energy beam from the beamsplitter. The second return mirror reflects the second beam of energy to the retroreflector, to the planar optical surface of the moving mirror and to the beamsplitter. The beamsplitter combines at least a part of each of the first and second energy beams to form a combined beam so as to facilitate transmitting the combined beam to a detector of the spectrometer.
The spectrometer of the present invention can use a retroreflector made from a cube-corner reflector, a lateral-transfer retroreflector, a roof reflector, or any other equivalent reflecting means. It is preferred that the retroreflector inverts the energy beam from the moving mirror. Path length variation can be achieved by rotating the moving mirror about an axis of rotation, by pivoting the mirror about a pivoting axis, by sliding the mirror, by translating the mirror, or by any equivalent means for displacing the mirror to create the path length variation needed for the interferometic effect needed for obtaining spectroscopic or metrologic measurements.
It is a feature of the present invention that the path length of an interferometer can change fast by moving a single mirror through any combination of tilting, rotating, oscillating, precessing, nutating or translating motions without disturbing the optical alignment of the spectrometer. The mirrors that control the optical alignment of the interferometer remain fixed. The moving mirror of the present invention has the ability to move fast without disturbing the accuracy of its interferometric effect. The present invention can measure spectra rapidly. Thus, the present invention provides an elegantly simple and economical way to spectroscopically analyze ephemeral, transient phenomena--even ones that before have been difficult, or even impossible, to measure economically.
One feature of the present invention is that the moving mirror can be a relatively simple, flat mirror. If moved by rotation, structural dynamics favor having the moving mirror in the shape of a disk-shaped, approximately flat, planar mirror. In this way the moving mirror can be made lighter and more resistant to deformation under the acceleration of motion. Further, fabrication and mounting of the moving mirror can be simplified by requiring optical access to only one side of the moving mirror. The two sides of the moving mirror need not be parallel, and only one side requires an optical finish. Moreover, the side of the moving mirror that does not have an optical finish can be contoured to better enable the moving mirror to withstand the dynamic stresses induced by rapid motion.
One of the preferred embodiments of the present invention uses complementary reflections from a single plane mirror to cancel angular variation, caused by tilt, of a beam reflecting from that surface. This feature enables the present invention to achieve rapid scan rates in a mechanically simple and economical spectrometer.
All of these objectives, features and advantages of the present invention, and more, are illustrated below in the drawings and detailed description that follows.